The invention relates to a process for monitoring the potential with respect to the soil of a buried metallic structure that is cathodically protected by a DC voltage applied thereto. A measuring electrode is inserted into the soil near the cathodically protected structure. This measuring electrode is electrically connected to the metallic structure and to a measuring circuit for measuring the potential difference between the measuring electrode and a reference electrode situated on or in the soil.
Depending on the type of soil in which a cathodically protected metal structure, such as a steel pipeline, is situated, the potential of the structure with respect to the soil must be lower than -850 mV to -950 mV in order to prevent corrosion. In order to ensure good operation of the cathodic protection system, one should check this potential regularly.
A known arrangement for monitoring the cathodic protection potential of a buried metallic structure is described in German Patent Application laid open for public inspection No. 2707265 incorporated herein by reference. In that known arrangement the voltage difference between the protected structure and the reference electrode is not measured directly. Direct voltage measurement would yield an inaccurate result because of the voltage drop through the soil caused by the passage either of a current flow due to the applied cathodic protective voltage or by the passage of stray currents derived from nearby electrical installations. When, as indicated, use is made of a measuring electrode connected to the structure, the measuring electrode and the reference electrode can be so positioned with respect to one another that no extraneous voltage drop is produced between them by the passage of current through the soil. For example, a single measuring probe can contain both the measuring electrode and the reference electrode in close proximity to one another. In the measuring probe described in the above-mentioned German patent application No. 2707265, the measuring electrode is a flat ring through which the active end of a reference electrode projects.
For a regular check on the proper operation of the cathodic protection system, a measurement can be carried out from time to time in the manner described above. However, sometimes it is desirable to continuously monitor the potential so that the average potential over a relatively long period of time, for example a month, can be determined.